1. Field of the Invention
The present invention relates to ceramics suited for use as packages for accommodating semiconductor devices and as Insulating substrates for wiring boards such as multi-layer wiring boards. More specifically, the invention relates to low-temperature-fired ceramics having a large strength which can be co-fired together with a low-resistance conductor such as copper or silver, and which, particularly, makes it easy to adjust the dielectric constant and the coefficient of thermal expansion, to a method of preparing the same, as well as to a wiring board using the ceramics as an insulating substrate.
2. Description of the Prior Art
As the ceramic multi-layer wiring boards, there have heretofore been most widely used those obtained by forming a wiring layer of a high-melting metal such as tungsten or molybdenum on the surface or inside of the insulating substrate formed of a sintered article of alumina.
In modern era of highly sophisticated information technology, further, the frequency band which is allowed to be used is shifting toward the ever higher zones. In the high-frequency wiring boards that must transmit signals of such high frequencies, it is a requirement to decrease the resistance of the conductor forming the wiring layer from the standpoint of transmitting high-frequency signals without loss as well as to decrease the dielectric loss of the insulating substrate in the high-frequency regions.
However, the conventional high-melting metals such as tungsten (W) and molybdenum (Mo) have large resistances, transmit signals at slow speeds, and are not capable of transmitting signals in the high-frequency regions of not lower than 1 GHz. Therefore, it has been desired to use low-resistance metals such as copper, silver and gold in place of tungsten and molybdenum.
However, the metallized wiring layers of such low-resistance metals have low melting points and cannot be co-fired with alumina. In recent years, therefore, there has been developed a wiring board by using, as an insulating substrate, a so-called glass ceramics using a glass or a composite material of the glass and ceramics.
However, the conventional glass ceramics have a small strength and low mechanical reliability. Besides, with the insulating substrate formed of such ceramics, the wiring layer formed on the surface thereof is peeled off together with the insulating substrate due to tensile stress built up in the wiring layer, making it difficult to enhance the metallizing strength.
According to Japanese Unexamined Patent Publication (Kokai) No. 120436/1998 and 49531/1999, there have been proposed ceramics obtained by adding ceramic powders such as of alumina and mullite to a glass powder capable of precipitating a diopside crystal phase, and firing the mixture. There have been disclosed that the ceramics feature decreased dielectric losses in the microwave band, and strengths which are increased up to 2200 kg/cm2.
According to Japanese Unexamined Patent Publication (Kokai) No. 120436/1998 and 49531/1999, however, the ceramic strengths can be increased only up to about 2200 kg/cm2, and it has been desired to further increase the ceramic strengths from the standpoint of enhancing the metallizing strength of the wiring layer. As for the ceramics to which alumina and mullite are added as fillers, it is desired to decrease the dielectric constants and dielectric losses, since they exhibit large dielectric constants and dielectric losses in the high-frequency bands.
There has further been proposed a method of preparing ceramics by adding cordierite as a filler to a glass powder capable of precipitating a diopside crystal phase, and firing the mixture thereof leaving, however, a problem in that the ceramic density is not heightened due to poor wettability between the diopside crystal phase that is precipitated and the cordierite, and that the ceramic strength decreases due to an increase in the open porosity (e.g., not smaller than 2%).
It is therefore an object of the present invention to provide a low-temperature-fired ceramics which is made dense being fired at from 800 to 1050xc2x0 C. at which it can be co-fired together with a low-resistance conductor for forming a wiring layer such as of gold, silver or copper, the ceramics exhibiting good dielectric properties even in high-frequency regions and a large strength, a method of preparing the same, and a wiring board using the above ceramics.
The present inventors have discovered the fact that ceramics having a large strength can be obtained by molding a mixture of a glass capable of precipitating a diopside crystal phase and a glass capable of precipitating a cordierite crystal phase, and by firing the molded article under predetermined conditions, the ceramics capable of being co-fired with copper or silver and exhibiting excellent dielectric properties in a high-frequency band, and have thus arrived at the present invention.
According to the present invention, there is provided ceramics containing a diopside crystal phase and a cordierite phase, the remainder being a glass phase and/or other ceramic crystal phases, and having an open porosity of not larger than 1%.
In the ceramics of the present invention, it is desired that:
(a) The total content of the diopside crystal phase and the cordierite crystal phase is not smaller than 50% by weight;
(b) Both the diopside crystal phase and the cordierite crystal phase are those precipitated from the glasses; and
(c) The flexural strength is not smaller than 250 MPa, the Vickers"" hardness is not smaller than 650, the dielectric constant is not larger than 8, and the dielectric loss at 60 GHz is not larger than 20xc3x9710xe2x88x924.
According to the present invention, further, there is provided a method of preparing ceramics comprising:
preparing a mixed powder containing a first glass powder capable of precipitating a diopside crystal phase and a second glass powder capable of precipitating a cordierite crystal phase;
molding the mixed powder; and
firing the obtained molded article at a temperature of from 800 to 1050xc2x0 C., so that the diopside crystal phase and the cordierite crystal phase are precipitated from the first and second glasses, respectively.
According to the method of preparing ceramics of the present invention, it is desired that:
(d) Not smaller than 80% by weight of the first glass powder and the second glass powder are crystallized into the diopside crystal phase and the cordierite crystal phase;
(e) The firing is conducted in two steps of from 800 to 900xc2x0 C. and from 910 to 1000xc2x0 C.; and
(f) The mixed powder contains a ceramic powder that is not decomposed by the firing.
According to the present invention, there is further provided a wiring board comprising an insulating substrate provided with a wiring layer, the insulating substrate being formed of the ceramics.
In the wiring board of the present invention, the wiring layer is formed on the surface and/or inside of the insulating substrate, the wiring layer, desirably, containing copper or silver as a conducting component.